Tamper-evident pressure sensitive facestock labels

ABSTRACT

A film useful in tamper-evident pressure sensitive label applications comprises a blend of incompatible polymers wherein the film is melt extrusion cast and has a cohesive strength less than the adhesive strength of an adhesive material bonded to one surface of the film. The adhesive material has an adhesive strength greater than the cohesive strength of the film so that when the tamper-evident pressure sensitive label has been adhered to a substrate attempted tampering or removal of the label causes at least partial destruction of the film.

This is a divisional of application Ser. No. 07/887,319, filed on May22, 1992, now U.S. Pat. No. 5,277,971, issued Jan. 11, 1994.

TECHNICAL FIELD

This invention relates to films and in particular to films useful inmaking labels which readily disintegrate upon any evidence of tampering.

BACKGROUND

The present invention is directed to tamper-evident, pressure sensitivefacestock labels which are useful on a wide variety of substratesincluding for example, stainless steel, aluminum, paper, paper-likeproducts and plastics, such as polyethylene.

Pressure sensitive labels consist basically of three components: arelease liner, a pressure sensitive adhesive and a facestock material oractual label. The facestock label is printed on one side with theintended decoration design or message, while the opposite side of thelabel is coated with the designated pressure sensitive adhesivematerial. The function of the release liner is to protect the adhesive,act as a die-cutting base and carry the face stock label through aconverting process. Pressure sensitive labels are subjected to variousconverting processes or steps during their manufacture and use. Themajor converting processes include (1) coating one side of the releaseliner with a pressure sensitive adhesive, (2) laminating the releaseliner to the facestock material, (3) printing on the facestock side ofthe label, (4) die-cutting the facestock material, and (5) applying thefacestock label to the surface of an end use product, often known aslabel dispensing and application.

There have been many efforts to provide tamper-evident materials forapplication to packages for pharmaceutical and foodstuff products. Inaddition, various types of labels disintegrate or show evidence oftampering if the label is removed from the substrate.

The U.S. Pat. No. 5,013,088 to Marin discloses disintegratable maskinglabels which have a plurality of cuts in the label such that upontampering, portions of the label remain bonded to the substrate and theremaining label fragments show evidence of tampering of the label.

The U.S. Pat. No. 4,931,327 to Liu et al. discloses a tamper evidentfilm comprising a core layer and at least one oriented tamper-evidentlayer applied to the core layer. The tamper-evident layer comprises amixture of a polyolefin and filler or particles which induce cavitationupon orientation of the film. An oriented skin layer is applied to thecore layer if tamper-evident layers have not previously been applied toboth sides of the core layer.

The U.S. Pat. No. 4,876,123 to Rivera et al. discloses a tamperindicating tape including a delaminating film which delaminatesinternally between first and second major surfaces. The tamperindicating tape also includes colored indicia printed on the majorsurfaces of the film and an adhesive means for adhering the film to asubstrate.

The U.S. Pat. No. 4,121,003 to Williams discloses tamper indicatinglabels comprising a laminate of a transparent outer sheet having atamper-indicated, information-containing pattern printed on its innersurface and a pressure sensitive adhesive film coated on the printedinner surface of the outer sheet.

In addition, various types of tamper-evident labels are made frompolyvinylchloride and cellulose acetate materials. Typically, thesetamper-evident films are made using a solution casting process. Thesolution casting of polyvinylchloride and/or cellulose acetate film isexpensive and complicated and requires extensive handling of thesolvent. A polyvinylchloride and/or cellulose acetate material isdissolved in a suitable solvent and thereafter spread out over drums,belts, coated papers or the like to evaporate the solvents. Uponevaporation of the solvent, the solute loses its flexibility andeventually becomes very rigid and brittle. The rigid, brittle materialis easily susceptible to destruction if any external mechanical stressesare exerted.

Until the present invention it has not been possible to use meltextrusion processes, blown or cast, for producing brittle type films. Ina typical cast extrusion process the film undergoes tension and stressand is subject to movement and vibrations of the processing equipmentwhich causes the film to shatter or fracture.

Therefore, it is an object of the present invention to provide atamper-evident pressure sensitive coated facestock label which providesvisible evidence that tampering has occurred.

It is a further object of the present invention to providetamper-evident pressure sensitive coated facestock labels which can beapplied on diverse types of substrates such as metals, plastics, paper,paper-like products and the like.

It is still a further object of the present invention to provide atamper-evident pressure sensitive coated facestock label which readilydisintegrates upon removal of the label from a substrate.

It is an additional object of the present invention to provide atamper-evident pressure sensitive label which does not involve the useof solvents during the manufacturing process of the label.

It is an additional object of the present invention to provide atamper-evident pressure sensitive label which can be produced using acast extrusion process.

It is still an additional object of the present invention to provide amethod for producing a tamper-evident pressure sensitive label by castextrusion.

DISCLOSURE OF INVENTION

The present invention provides a tamper-evident, pressure sensitivecoated facestock label having the required destructive brittleness ordestructibility needed to show evidence of tampering. The pressuresensitive coated facestock label includes a film comprised of a blend oftwo incompatible polymeric systems. The first polymeric system comprisesa polymer which can be processed into a readily handleable film. Thesecond polymeric system comprises a polymer which is normally toobrittle to be processed into a film using a melt extrusion process.

Until the present invention, it was not possible to melt extrude abrittle film. According to the present invention, the film cansurprisingly be made using a melt extrusion casting process and does notrequire the use of any solvents during the film manufacturing process.The film of the present invention can be made such that one side ismatte embossed and the other side is smooth or glossy. The smooth sideprovides a good surface for most types of label printing applicationsand for applications requiring good image transfer such as a thermaltransfer printing coating, for example applying barcode data. The filmcan be converted to a pressure sensitive coated facestock label usingpressure sensitive coating and lamination equipment. A suitable adhesivematerial is applied to the film to form the tamper-evident pressuresensitive coated facestock label.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional schematic representation of one embodimentof the label of the present invention with a release liner in place.

FIG. 1B is a cross-sectional schematic representation of the label shownin FIG. 1A after application to a substrate and attempted removal of thelabel.

FIG. 2A is a top view of one embodiment of a label of the presentinvention applied to a substrate.

FIG. 2B is a top view of the label shown in FIG. 2A after attemptedremoval from the substrate.

FIG. 3A is a schematic diagram showing a side elevation of one side of acast extrusion equipment at an early stage of producing film.

FIG. 3B is a schematic diagram showing a side elevation of one side ofcast extrusion equipment at a later stage of producing film.

FIG. 4A is a schematic diagram shown in perspective of a portion of aribbon removal system.

FIG. 4B is a schematic diagram shown in perspective of a portion of aribbon removal system.

FIG. 4C is a schematic diagram shown in perspective of one embodiment ofa portion of a ribbon removal system according to the present invention.

BEST MODE FOR CARRYING OUT INVENTION

Referring now to the drawings, there is shown in FIGS. 1A, 1B and 2A,2B, a tamper-evident, pressure sensitive label according to the presentinvention and generally designated by the reference numeral 10. Thelabel generally includes a film 12 having an upper surface 14 and alower surface 16 and an adhesive layer 20. The film comprises acomposition of a blend of two incompatible polymeric systems. The filmhas desirable poor physical properties such that the film readilydestructs or tears upon attempted removal from a substrate 24. Theadhesive material 20 is applied to the lower surface 16 of the blendedfilm. The desired information can be printed on the top surface 14 ofthe film 12. Depending upon the desired characteristics of the label,the top surface can either have a matte or smooth or glossy surface. Thesmooth side of the film provides a good surface for most types of labelprinting applications. Most printing applications require a good imagetransfer such as in thermal transfer printing used for barcodingapplications. Various types of printing inks or material are useful withthe film of the present invention. The exact chemical composition of theprinting material depends upon the end use application and various knownprinting materials can be employed as are known in the art. Again,depending upon the end use application, the printed material may beapplied to the film after application of an adhesive material thereto.

The adhesive material used depends upon the end use application andwhich type of substrate material the label will be applied to. Theadhesive material has an adhesive strength such that when the label isattempted to be removed from the substrate the adhesive material readilyallows the film material to tear. In one preferred embodiment theadhesive material comprises a strong acrylic or rubber-based pressuresensitive adhesive system. In accordance with the present invention, theadhesive bond strength of the adhesive material is greater than theinternal cohesive strength of the film. Thus, when a mechanical stressis applied to the tamper-evident label, the film tears or is destructedsuch that, while a portion of the label can be removed, the remainingportion remains adhered to the substrate. As a result of thisdestruction, the label is incapable of being reapplied or resealedwithout showing evidence of tampering thereof. As such, an adhesivematerial is selected so that the adhesive material possesses the desiredadhesive bond strength to bond the film to the substrate. Thus, labelsapplied to such surfaces as a plastic molded computer housing mayrequire a different adhesive strength than labels to be applied to othersurfaces such as stainless steel, aluminum or other type of smoothsurface. It is noted that, in various end use applications, at least aportion of the adhesive material may remain adhered to the substrateupon the attempted removal of the label from the substrate. The adhesivematerial has a greater adhesive strength than the cohesive strength ofthe film material such that the film material readily tears or breaksdue to its poor physical properties. Thus, as can be seen in FIG. 1B, aportion 18 of the film 12 can be detached from the substrate 24 suchthat a remaining portion 19 of the film 12 remains adhered to thesubstrate 24.

As can be further seen in FIGS. 2A and 2B, a label 10 is applied to asubstrate 24. Upon attempted removal thereof, as shown in FIG. 2B, thelabel 10 tears due to its brittleness or easily destructible physicalproperties. In certain embodiments, the adhesive material 20 can remainadhered to the substrate 24.

The thickness of the film and the thickness of the adhesive materialdepend upon the properties desired for a particular end use labelapplication. Typically, the film can have a thickness of between about1.0 mil. and about 5 mils, while the adhesive material be applied to athickness between about 0.5 mils and about 1.0 mils.

Referring again to FIG. 1A, the label 10 can be temporarily adhered to arelease liner 26 so that the label can be easily formed. Various typesof release liners including for example, a silicon coated paper or filmcan be utilized in the present invention. The release liner 26 isselected such that the label can be readily removed therefrom and yetthe label remains adhered thereto during any storage or shipment of thelabels. The adhesive bond strength between the adhesive material 20 andthe release liner 26 is lower than the adhesive bond strength betweenthe adhesive material 20 and the film 12 such that the release liner 26can be readily peeled from the adhesive material 20 and the film 12 doesnot tear or destruct upon removable from the release liner 26.

The film of the present invention comprises a blend of two or moreincompatible polymers. The first polymer comprises a polymer which canbe made into a readily handleable film using a melt extrusion process.The second polymer comprises a brittle polymer which cannot be meltextrusion cast or processed by itself into a film. When a brittlepolymer (such as polystyrene or the like) is blended with anincompatible, but readily processable or handleable polymer, a film canbe produced using a melt extrusion process. The film comprises a blendof the readily handleable first polymer and incompatible brittle secondpolymer wherein there are weak interphase boundaries between the firstpolymer and the second polymer. It is believed that the presence ofthese weak boundaries allows the film to have brittle or "poor" physicalproperties. These "poor" physical properties, which include lowelongation, and tensile strength, allow the film to be readily torn ordestructed. The film readily tears in both the transverse and machinedirections. The film is sufficiently brittle to be useful intamper-evident applications and yet has enough strength to be initiallycast and processed using a melt extrusion process. The film also hassufficient strength to be further processed into a label usingpressure-sensitive coating and laminating processes.

The first polymer includes from about 40% to about 70%, by weight, of atleast one polymer such as polyolefin, including for example,polyethylene (including low, medium and high density polyethylene),polypropylene, copolymers of polyethylene, copolymers of polypropyleneand the like; blends of the above polymers and the like. In certainembodiments, the first polymer comprises about 50% to about 70%, byweight, high density polyethylene having a density of about 0.94 toabout 0.965 g/cc. and in other embodiments, the first polymer comprisesabout 60% to about 70%, by weight, high density polyethylene.

The second polymer includes from about 30% to about 60%, by weight, ofat least one stiff or brittle type polymer including, for example, fromsuch polymers as polystyrene, polymethylmethacrylate, blends thereof andthe like. In one embodiment of the present invention, the second polymercomprises about 30% to about 60%, by weight, polystyrene. In certainembodiments, the second polymer comprises about 30% to about 50%, byweight, polystyrene and in other embodiments about 30% to about 40%, byweight, polystyrene.

The film made according to the present invention can be natural or whitein color. It is also within the contemplated scope of the presentinvention that various other additives and fillers known in the industrycan be added to the film. In addition, colorants can be added to thefilm to impart the desired color.

The film produced according to the present invention can be processedinto pressure sensitive labels and can receive suitable pressuresensitive adhesives. The film can be corona treated, as is known in theart, such that the surfaces of the film readily accept any printing inkor a top coating and readily receive any pressure sensitive adhesivematerials. The film of the present invention can be produced having amatte finish on one side of the film and a smooth or glossy finish onthe other side of the film. It is contemplated that the smooth side ofthe film can have a smoothness in the range of about 0.25 to about 10Ra. This connection Ra is defined as the arithmetic average height ofthe micro peaks to the centerline of a particular finish surface, asmeasured by a profilometer. This measurement is usually expressed in10⁻⁶ microinches. The one-side matte label of the present invention canbe printed upon either the matte finish side or the smooth finish side.Thus, the one-side matte film provides the label manufacturer withgreater versatility. In addition, the label manufacturers need onlystock one type of film, thereby decreasing film inventory andcomplications of logistics and production methods.

The same "poor" physical property characteristics which make the filmespecially useful in tamper-evident applications, provide problems whenmanufacturing such film. Surprisingly, the film of the present inventioncan be made using a melt extrusion process. The film of the presentinvention may be preferably produced by dry blending the first andsecond polymers together. The blended polymers are then cast using amelt extrusion process.

A schematic diagram showing suitable film extrusion equipment 28 forproducing the film of the present invention is shown in FIGS. 3A and 3B.The polymers are blended together with other, optional ingredients (notshown) and fed into an extruder 30. The blended polymers are extrudedfrom the extruder 30 to a die 32. The extruded polymer is cast into niprolls 34 which form an embossed film 36. The film 36 passes through agage 35 and a film treater assembly 39 which provides a corona treatmentto the film 36. The film 36 is advanced through the film extrusionequipment by a plurality of rolls 38 to a winder assembly 40.

Lateral or outer edges ("ribbons") 44 of the film 36 are trimmed fromthe extruded film during the film extrusion process by slitters orblades 42. These ribbons 44, are recycled back (not shown) into theextruder 30.

In a typical film cast extrusion process the film is extruded from thedie 32 and passed or laced through the different rolls 38 until the filmis wound using the winder assembly 40. However, since the film of thepresent invention is very brittle, it is difficult and time consuming tolace the film through the film extrusion equipment. The film tends tobreak into pieces if any pulling forces are exerted on the film.

During the initial start-up of the process for producing the film of thepresent invention the film 36 is laced through the film extrusionequipment using a carrier or lace-up film 37, as seen in FIG. 3A. Thecarrier film 37 is fed through the nip rolls 34 prior to or with aleading edge (not shown) of the film 36 being extruded from the die 32.The film 36 and the carrier film 37 are in contact. The carrier film 37aids in carrying or holding the film 36 as it is initially fed or lacedthrough the film extrusion equipment such that no damage, cracking orbreaking of the film 36 occurs.

The carrier film 37 carries the leading edge (not shown) of the film 36to the winder assembly 40. After the leading edge of the film 36 andcarrier film 37 are laced through the extrusion equipment, the carrierfilm 37 is then cut with a blade 33, as seen in FIG. 3B to leave onlythe film 36 running through the film extrusion equipment. In certainembodiments a portion of the carrier film 37 is wound by the winderassembly 40 on a roll along with the film 36. The result is that thefilm 36 then proceeds through the film extrusion equipment. Suitablecarrier films are those films which are flexible and have suitablestrength. For example, polyethylene films and low density polyethylenefilms are flexible, tough and are examples of a film useful to lacethrough film extrusion equipment.

FIG. 4A shows a portion of a ribbon removal system wherein the lateraledges or ribbons 44 of the film 36 are cut by razor-like blades 42 toachieve the desired film width. The ribbons 44 are passed over an idlerroller 45 which is driven by the ribbons 44. Ribbon suction tubes 46 and47, which are spaced apart from the idler roll 45 receive the ribbons 44from each edge of the film 36. As can be seen in FIG. 4A, the ribbons 44pass over an idler roll 45 across a distance of at least a couple offeet or more to the suction tubes 46 and 47. The ribbon suction tubes 46and 47 typically have a relatively large diameter of approximately 4inches or more. The ribbon suction tubes 46 and 47 are attached to ablower assembly 48 which vacuums or sucks the ribbons 44 and returns(not shown) the ribbon 44 to the extruder 30.

However, when the brittle films of the present invention are cut, theareas where the ribbon cutting blades are inserted constitute highlyconcentrated stress points or very weak points. The ribbon removalsystem described above is unable to efficiently remove the ribbons beinggenerated from the film 36 thus causing back-up of the ribbon in thesuction tubes which then causes film breakage. This problem is furthercompounded by the paper-like nature of the film 36 which causes theribbons to drag very slowly or even stop completely in the suction tubes(not shown).

In addition, during the extrusion of the film, the winder assembly 40oscillates laterally in order to produce a uniform roll conformation(not shown). The oscillation levels out any gage variations in the film.As the film web oscillates laterally (not shown) the ribbon 44 becomesnarrower on one side of the film and wider on the other side. Duringcast extrusion processes with films which are flexible, the films willfold and bend such that a ribbon which is wider than the opening of thesuction tube will enter the tube without problems. However, the brittlefilm of the present invention will not bend or fold without causingproblems since there is high friction between the brittle or paper-likenature of the film 36 and the walls of the suction tubes. Thus, as seenin FIG. 4B, when the oscillation causes the ribbon width to exceed theopening of suction tube 47, the ribbon 44 wrinkles. The film 36 is cutinto ribbons 44 in the machine direction. Due to the nature of the blendand orientation of the film, the film is weaker in the transversedirection than in the machine direction. When the blades 42 cut the filmin the machine direction the ribbons 44 easily separate from the film36. The points of initiation where the ribbons are cut by the blades 42are areas of high stress on the film and the film is subject to crackingor breaking at that point. In addition, the ribbons 44 being cut tend toslightly vibrate close to the blades 42 if the blower assembly 48 doesnot remove or vacuum the ribbons 44 efficiently and at a constanttension. These vibrations cause slack or bagginess in the ribbons 44.Slack in the ribbons 44 at the points where the blades 42 cut theribbons 44 causes the film to break. These problems are compounded bythe facts that the ribbons 44 are made of a stiff or brittle materialand that the width of the ribbons 44 varies as the film 36 is beingmade.

It has been found that it is not a solution to widen the width of thesuction tube opening to accommodate the corresponding width of theribbon being generated by the oscillating winder assembly 40. Highvelocity air requirements are necessary in order to move the ribbonsthrough the suction tubes, and it is necessary that the openings of thesuction tubes be narrow in order to maintain the high velocitiesrequired to remove the ribbons at an appropriate rate. A larger openingof the air inlet on the suction tube will reduce the velocities andcreates the aforementioned difficulties.

The inventors have surprisingly found a way to overcome the ribbonhandling problems by maintaining constant tension on the ribbons,thereby preventing film breakage. According to the present invention, itis possible to remove the ribbons in a manner such that the film doesnot break.

Referring now to FIG. 4C, the film web 36 is passed over a roll 38 andis cut by the slitter blades 42. The film 36 and ribbons 44 are pulledby a driven roll 55. In certain embodiments it is preferred that thesurface of the driven roll 55 have a "rubber" type surface in order tobetter pull or carry the film 36 and ribbons 44. Suction tubes 56 and 57are adjacent the driven roll 55 and, in certain embodiments, the tubes56 and 57 are a few inches from the driven roll 55. Thereafter, eachsuction tube 56 and 57 is individually connected to blower assemblies 58and 59, respectively. It is advantageous to provide an individual blowerconnected to each suction tube. It is also advantageous to providesuction tubes with a small diameter in order to achieve a higher airvelocity moving through the suction tubes. The higher air velocity keepsthe ribbons moving through the tubes and prevents the ribbons fromdragging or becoming stuck in the tubes. As the ribbon 44 is beingremoved by the suction tubes 56 and 57 the rate of suction from theblower assemblies 58 and 59 can be adjusted to remove the ribbons 44 ata rate of speed which controls tension on ribbons and prevents anybagginess or slack in the ribbons. In addition, the driven roll 55 aidsin maintaining the appropriate tension on the ribbon 44. It has furtherbeen found that when the tubes 56 and 57 are positioned close to oradjacent the driven roll 55, there is less tension on and vibration ofthe moving ribbons. Thus, the removal of the ribbon at a preferred rateof speed allows the tension on the ribbons to be adjusted so that nobreakage occurs to the extruded film.

In addition, an electric eye system 60 can be used which senses theribbon width. When the width of the ribbon exceeds a predeterminedwidth, the electric eye system 60 is triggered (by sensing the excessribbon width). The electric eye system 60 sends a signal to activatesecond slitter blades 62. The second blades 62 cut the ribbon into twoportions, 44A and 44B. What would have been, for example, a wide ribbontrying to crumple itself into a narrow opening, now becomes two narrowerribbons 44A and 44B which can overlap and fit into the opening of thesuction tube without causing problems.

Referring again to FIGS. 3A and 3B, when a target length of film iscollected on one roll, a new roll of film is needed. This change inrolls is called indexing. During the indexing step, the winder assembly40 rotates the film, and a large knife 64 is engaged to cut a trailingedge (not shown) of the film. The winder assembly 40 typically exertshigh tension forces on the film to prevent any bagginess in the filmduring indexing. However, since film of the present invention is brittleand is not very elastic, this mode of indexing causes the film to break.In addition, vibrations and movements occur during the movement androtation of the winder assembly and cutting action of the knife. Allthese strong vibrations can also cause the film web to break. However,the inventors have surprisingly found that in order to overcome theseproblems, the winder assembly can be adjusted to put less tension on thefilm as it is being indexed. The reduced tension on the film allows thetrailing edge of the film to be cut without causing the film to break.

The film of the present invention has transverse direction (TD) fracturecharacteristics which are different from the fracture characteristics inthe machine direction (MD). In the TD, the film fractures in azigzag-like manner. In the MD, the fracture propagates in a sharp, cleanand straight-like path. It is believed that these fracturecharacteristics are due to the blend morphology and orientation effectsinduced during cast extrusion of the film. The film of the presentinvention exhibits sufficient strength such that conventional top finishor pressure sensitive adhesive coating methods may be utilized to applythe adhesive thereto. Thereafter, the adhesive coated film can belaminated to the pressure sensitive methods may be utilized to apply theadhesive thereto. Thereafter, the adhesive coated film can be laminatedto the pressure sensitive release liner without damage due to breakingor cracking of the film.

INDUSTRIAL APPLICABILITY

The following examples illustrate the present invention. In all examplesthe tamper-evident pressure sensitive labels made according to thepresent invention were tested for destructibility.

EXAMPLE #1

A one-side matte extruded cast film was made as described above, andcomprised about 60% high density polyethylene and 40% polystyrene(weight %). The physical properties of the film follow, where MD is themachine direction and TD is the transverse direction:

    ______________________________________                                        "Thickness by weight" Gauge (mils):                                                                          2.18                                           Tensile strength (lbs)  MD:    1.01                                                                   TD:    0.59                                           Elongation (%)          MD:    5.05                                                                   TD:    2.95                                           ______________________________________                                    

Above film was coated with a strong pressure sensitive adhesive materialto form labels. Thereafter, the labels were applied on differentsubstrates and tested for their destructibility or brittleness afterdifferent pre-set times: immediately, 1 minute, 15 minutes, 1 hour, 24hours. Destructibility was scaled from 1 to 5; 1 being not destructibleand 5 being completely destructible. The following results wereobtained:

    ______________________________________                                        Substrate  Immediately    1 min.  15 min.                                     ______________________________________                                        Stainless steel                                                                          4              4       4                                           Aluminum   3              4       3                                           Polyethylene                                                                             3              4       3                                           Stainless steel                                                                          4              4                                                   Aluminum   4              4                                                   Polyethylene                                                                             4              4                                                   ______________________________________                                    

EXAMPLE #2

A one-side matte extruded cast film was made of the following: about 50%high density polyethylene and 50% polystyrene (weight %), yielded thefollowing physical properties:

    ______________________________________                                        "Thickness by weight" Gauge (mils):                                                                          2.12                                           Tensile strength (lbs)  MD:    1.11                                                                   TD:    1.29                                           Elongation (%)          MD:    5.55                                                                   TD:    6.47                                           ______________________________________                                    

In a manner similar to Example #1, the above film was coated with astrong pressure sensitive adhesive material to form labels which weretested for destructibility. The following results were obtained:

    ______________________________________                                        Substrate                                                                     ______________________________________                                                   Immediately   1 min.   15 min.                                     Stainless steel                                                                          4             4        4                                           Aluminum   4             3        4                                           Polyethylene                                                                             4             4        4                                                      1 hour        24 hours                                             Stainless steel                                                                          4             4.5                                                  Aluminum   4             4.5                                                  Polyethylene                                                                             4             4.5                                                  ______________________________________                                    

As can be seen from the above Examples 1 and 2, minimal self-integrityof the label film is achieved. The film exhibits the desired poorphysical qualities in its final form yet the film can be processed andhandled on conventional extruded film equipment and further converted toa label using conventional pressure sensitive coating and laminationprocesses.

While there has been described herein what is believed to be thepreferred embodiments of the present invention, those skilled in the artwill recognize that changes and modifications may be made theretowithout departing from the spirit of the invention, and it is intendedto claim all such changes and modifications as fall within the truescope of the invention.

We claim:
 1. A tamper-evident film suitable for use in forming atamper-evident pressure sensitive label having less cohesive strengththan the adhesive strength of an adhesive material bonded to one surfaceof the tamper-evident film, the tamper-evident film having been made bya process comprising the steps of extruding a mixture comprising anincompatible blend consisting of(a) about 40% to about 70%, by weight,of a first polymer consisting essentially of at least one readilyextrudable polyolefin; and (b) about 30% to about 60%, by weight, of asecond polymer consisting essentially of a brittle polymer selected fromthe group consisting of polystyrene, polymethylmethacrylate, or blendsthereof; through a die in the form of a film, and continuously extrudinga suitable length of film and withdrawing the tamper-evident film suchthat the tamper-evident film is not broken; the film having physicalproperties which allow the film to be readily torn or destroyed in boththe transverse and machine direction.
 2. The film of claim 1, whereinthe first polymer is polyethylene, polypropylene, copolymers ofpolyethylene, copolymers of polypropylene or blends thereof.
 3. The filmof claim 1, wherein the first polymer consists essentially ofapproximately 50% to about 70%, by weight, high density polyethylene andthe second polymer consists of about 30% to about 50%, by weight,polystyrene.
 4. The film of claim 1, wherein the film has a matte finishon a first surface and a smooth or glossy finish on a second surface. 5.The film of claim 1, wherein as the tamper-evident film is initiallyextruded from the die, a carrier film is used to lace the tamper-evidentfilm through film extrusion equipment.
 6. The film of claim 1, wherein,as the tamper-evident film is continuously extruded, lateral edges ofthe film are cut to form ribbons of the film which are removed at aconstant tension without causing breakage of the film.
 7. The film ofclaim 1, wherein, after a suitable length of the tamper-evident film isextruded and wound on a roll, reduced tension is placed on the film as atrailing edge of the film is being cut or indexed.
 8. A tamper-evidentfilm suitable for use in forming a tamper-evident pressure sensitivelabel having less cohesive strength than an adhesive material bonded toone surface of the tamper-evident film, the tamper-evident film havingbeen made by a process comprising the steps of extruding a mixturecomprising an incompatible resin blend consisting of(a) about 40% toabout 70%, by weight, of a first polymer consisting essentially of atleast one readily extrudable polyolefin; and (b) about 30% to about 60%,by weight, of a second polymer consisting essentially of a brittlepolymer selected from the group consisting of polystyrene,polymethylmethacrylate, or blends thereof; through a die in the form ofthe tamper-evident film, wherein as the tamper-evident film is initiallyextruded from the die, a carrier film is used to lace the tamper-evidentfilm through film extrusion equipment; and, continuously extruding asuitable length of film and withdrawing the tamper-evident film suchthat the tamper-evident film is not broken; the film having physicalproperties which allow the film to be readily torn or destroyed in boththe transverse and machine direction.
 9. The tamper-evident film ofclaim 8, wherein the first polymer is polyethylene, polypropylene,copolymers of polyethylene, copolymers of polypropylene or blendsthereof.
 10. The tamper-evident film of claim 8, wherein the firstpolymer consisting essentially of about 50% to about 70%, by weight,high density polyethylene and the second polymer comprises about 30% toabout 50%, by weight, polystyrene.
 11. The tamper-evident film of claim8, wherein the tamper-evident film has a matte finish on a first surfaceand a smooth or glossy finish on a second surface.
 12. Thetamper-evident film of claim 8, wherein the process further comprisescutting lateral edges or ribbons of film from the film as thetamper-evident film is continuously extruded, and removing the ribbonsof the tamper-evident film at a constant tension whereby thetamper-evident film is continuously extruded without causing breakage ofthe tamper-evident film.
 13. The tamper-evident film of claim 8, whereinthe process further comprises winding a suitable length of thetamper-evident film into a roll of film, and reducing tension on thetamper-evident film as a trailing edge of the film on the roll is beingcut or indexed.